This invention relates to the preparation of ceramic composites having a ceramic fibrous cloth or matt substrate or filament wound structure which has been impregnated and coated with a ceramic by a chemical vapor deposition process. The composites are particularly useful as components of heating furnaces or for other high temperature or wear applications. More particularly, the invention relates to applying nucleation sites in and on the fibrous substrate to achieve a more thorough infiltration of the substrate by the subsequent CVD coating. Silicon carbide and silicon nitride composites so produced are particularly useful as components of semiconductor diffusion furnaces.
In the overall manufacturing process for the production of semiconductors devices, such as diodes, transistors, and integrated circuits, a critical part of the process involves multiple steps of elevated temperature processing. The process involves oxidation and doping of thin silicon wafers interspersed with steps of etching of cavities or patterns on the wafer surfaces. The semiconductor devices may be made both separately and in an integrated circuit array. The oxidation steps and various doping and coating operations to which the silicon slices are subjected involve multiple heating and cooling cycles at temperatures in the range of from 400 to 1350.degree. C. These critical thermal processing steps generally take place in special electrically heated muffle furnaces. The silicon slices are generally placed in quartz, silicon carbide, silicon impregnated silicon carbide, or polysilicon boats, jigs, or fixtures which are then placed within a process tube of the muffle furnace so that the silicon slices can be processed through a predetermined time/temperature/atmosphere cycle. The atmosphere is carefully controlled and gases are usually fed into the necked-down end of the diffusion furnace process tube. In the process the silicon slices in the boats are typically supported on a paddle.
The components and process tubes used in the process must have excellent thermal shock resistance in order to permit the rapid heating to, and rapid cooling from, temperatures in the order of about 400.degree. C. to about 1350.degree. C. back to room temperature. The components and other furnace parts must also be of high mechanical strength at both elevated and room temperatures, have the ability to retain their shapes through a large number of heating and cooling cycles, not outgas, i.e. introduce any undesirable impurities into the process atmosphere during elevated temperature operations, and not introduce any dust-like contamination. Cleanliness and control of impurities are extremely important to producing semiconductor devices having the desired electrical characteristics.
The demanding conditions severely limit the materials which can successfully be used to fabricate diffusion furnace parts or components. Generally, a furnace consists of an external furnace liner which fits in the annular space between a heating element and a process tube; the process tube which fits into the liner and has a necked-down end for the introduction of the desired atmosphere; and a paddle, either as a wheeled carrier or as a cantilevered support, upon which are placed what are known as "wafer supports" or "boats". Occasionally an "internal" liner is used inside the process tube, particularly for processes involving progressive build-up of deposits. This internal liner can be tailored to have desirable properties and generally is designed to be replaced without having to replace the process tube. An alternative furnace configuration entails having the outermost tube as the process tube and containing an inner tube which also is a liner. Thus there can be external or outer liners and inner liners. Whenever the term "liner" is used herein, it is meant to include both external and internal liners unless one of the two is indicated. The process tube, paddle, and boat, have often been made of fused silica quartz while the liner has sometimes been composed of mullite or alumina. However, the silica components have been known to lose their mechanical strength and progressively devitrify with time at the processing temperatures involved. In addition, quartz components are very susceptible to extreme distortion due to the cyclic heating and cooling and also do not long withstand the frequent hydrofluoric acid cleaning which is normally required to maintain the necessary ultra-pure furnace environment. In a more recent modification of the process, the furnace liner, i.e. the tube which surrounds and supports the process tube, has been formed from silicon carbide, instead of mullite and alumina, and used with a quartz process tube or inner tube. Silicon carbide possesses high thermal conductivity and high strength as compared to the other materials and furthermore provides a barrier to sodium and other metallic ions coming from the heating element and related components. However, the initial silicon carbide bodies used were porous and permeable and therefore could not provide the controlled atmosphere and high purity environment required for many semiconductor manufacturing processes. U.S. Pat. No. 3,951,587 discloses furnace components composed of silicon carbide that are at least 99% pure and which are then impregnated with silicon which is at least 99.9% pure. Due to the high strength, imperviousness, and purity of this composition, it could be used as a process tube without need of a separate liner which saved space and improved overall purity and dependability. However, there are some operations where the free silicon is a problem.
One attempted solution to these problems in disclosed in U.S. Pat. No. 4,766,013 which discloses the use of a fibrous substrate which is carbonized to form a layer of pyrolytic carbon on the fibers and then the carbon-coated fibers are impregnated with, among others, silicon carbide by a chemical vapor deposition (CVD) procedure. The fibrous substrate may be silicon carbide fibers. The pyrolytic carbon coating is required to enable the fibers to be free to move relative to the CVD coating.
It is a principal object of the present invention to provide ceramic composites suitable for use as diffusion furnace components viz. liners and/or process tube, paddle, and boat, which have reduced porosity, superior oxidation resistance, thermal shock resistance, increased strength, the ability to retain their shape and composition through a large number of heating and cooling cycles, and improved impermeability to gases.
It is a further object to produce ceramic materials having a density of greater than about 70%, more preferably greater than 80%, and most preferably greater than about 85% of the theoretical density by a chemical vapor deposition of a ceramic onto fibrous ceramic substrates.